The invention relates to the field of fibre web dewatering. In particular, the invention relates to a long pressing zone press for the treatment of a fibre web.
The dewatering of a fibre web is usually performed by means of roller presses by leading the web via a press zone formed by two rolls, i.e. through a press nip. The web runs through the nip between press felts, and the felts carry away the water squeezed out of the web.
In a nip formed by two rolls, the greatest press force is reached as a narrow peak in the middle of the nip. This is disadvantageous both to the dewatering process and to the service life of the felts because the pressing is of very short duration at high speeds and great stress is put on the felts. Therefore, different kinds of so-called long nip presses have been constructed wherein one roll can be substituted by a concave counterpart, a press shoe. On the shoe side, to the rotary motion of the roll is usually matched by a watertight, endless loop of fabric, a press belt, that follows and glides on the surface of the lubricated counterpart. The press belt is supported by separate rolls, or alternatively the counterpart or the support of the shoe are made in such a form that a short belt can glide around it, the lubrication being arranged on the inside of the endless loop formed by the belt. By the use of shoe presses, nips are achieved whose effective length can be approximately 250-310 mm, depending on the size of the roll. In addition to roll/shoe combinations, patent documents mentioned below disclose presses composed of two opposite shoes.
In order to obtain an optimal dewatering effect, it is desirable that the press force can be adjusted within the nip zone. The incoming web, having a low dry solids content, cannot initially take a high pressure without breaking, and a compression force profile of the wrong shape can cause quality problems in the web if, for example, the water cannot be properly absorbed by the felt, but channels in the web. In order to achieve the final desired degree of dryness, the pressure peak has to be provided at the end of the pressing stage.
Many different approaches have been taken to obtain this result. Patent application CH5152/86 discloses a press shoe. On its surface, under the press belt, are provided cavities that serve as hydrostatic pressure chambers when hydraulic fluid is led into them through channels formed in the shoe. In addition to pressure, a stepwise heat treatment is achieved for the web by individually regulating the temperature of the hydraulic fluid in the separate chambers. Patent application FI 896163 also discloses a press shoe having a plurality of pockets on its sliding surface for feeding an individually pressurised lubricant, the pressure profile being adjustable in the nip zone for desired pressing and web speed conditions. Swedish patent application SE 9103823-2 discloses a press shoe provided with a pressure pocket, wherein the depth profile of the pressure pocket results in a combination of hydrostatic and hydrodynamic effect for obtaining a desired compression pressure profile. Swedish patent application SE 9201497-6, in addition to a profiled pressure pocket, comprises a heat control system for the front edge and for the back edge to prevent torsional forces from being generated as a result of temperature differences due to friction.
Further attempts have been made to affect the compression pressure profile by adjusting the tilt angles of the shoe. In the last-mentioned Swedish patent application 9201497-6, for example, is described how the front and the back edges of the shoe are supported by individually regulated hydraulic cylinders. German patent application DE 4113623 discloses a press shoe attached to a shoe base that together with its supporting framework forms a hydraulic piston/cylinder construction that is not only radially movable but also tiltable. Swedish patent application SE 8801933-6 also discloses a press shoe construction comprising in addition to a hydrostatic pressure pocket, lines of jacks under the front and back edges of the shoe, enabling tilting.
According to Finnish patent FI 70952, the compression pressure profile is adjusted in the different zones of the shoe surface by mounting separately controlled pressing elements in the surface of the press shoe. In addition, hydrostatic pressure chambers that have a direct influence on the press belt are arranged between the pressing elements, on the surface of the press shoe. Thus, the pressing elements serve as edge seals for the pressure chambers. In an embodiment according to Finnish patent FI 71369, the surface of the shoe is constituted by a sliding plate, one or more pressure hoses being arranged under the plate and mounted in the shoe.
The objective is to reach a maximum compression force of about 8 MPa. This requires great rigidity of the roll of a long nip press. In connection with shoe presses, the roll is prevented from bending by employing compensating constructions, a line of hydraulic regulating cylinders and sliding bearings, inside the roll. Such a design is, however, quite expensive and mechanically complicated.
The object of the present invention is to provide a simple long nip press construction that is suitable for the dewatering of pulp and paper webs and that comprises a press zone provided with a number of adjustment possibilities. The solution is obtained through the construction according to claim 1, comprising a roll and a concave counterpart whose sliding face is constituted by a press plate that, on the side of the incoming web, is anchored in the counterpart at one edge, the other edge of the plate being movably mounted in the counterpart. Under the press plate, in the surface of the counterpart, are provided hydraulic pressure chambers. The seals between these are constituted by flexible pressure pipes arranged perpendicularly to the web direction. Thus, the press plate forms a long nip by partly wrapping the web round the backing roll. No actual press shoe is used, as a result of which no hydraulic cylinder control systems, typical of shoe presses for controlling the position and the radial location of a shoe, are required. Accordingly, the structure of the press is considerably straightforward but provides sufficient possibilities to control and to differentiate the pressing force across the whole nip zone, and consequently, to influence the properties of the pulp web leaving the press.